Strengthened Alpha Brass and Method for Manufacturing the Same

ABSTRACT

An object of the present invention is to provide a strengthened alpha brass having a good balance between high offset yield strength and formability without deteriorated stress relaxation resistance in comparison with conventional brass and a manufacturing method of the strengthened alpha brass. In order to achieve this object, a strengthened alpha brass having a composition of 63 wt % to 75 wt % copper, incidental impurities and the balance zinc; the strengthened alpha brass which is obtained by using a starting plate material subjected to a re-crystallization annealing to have a grain size from 1-micron meter to 2-micron meter followed by cold rolling in 5% to 40% reduction, then the plate material is low temperature annealed at a temperature equal to or higher than the temperature at which a 0.2% offset yield strength exhibits a maximum value to adjust the 0.2% offset yield strength ([Sigma] 0.2 : MPa) to be equal to or higher than 90% of its maximum value is adopted. The strengthened alpha brass has a 0.2% offset yield strength of 450 MPa to 750 MPa and [minimum bend radius (MBR)]/[plate thickness (t)] and [0.2% offset yield strength] satisfy the following formula 
         MBR/t ≦0.0125×σ 0.2 −6.7 (σ 0.2 : 0.2% offset yield strength) 
     , and [Erichsen value (Er: mm)] and [ 0.2 % offset yield strength] preferably satisfy the following formula. 
         Er ≧−0.011×σ 0.2 +13.7 (σ 0.2 : 0.2% offset yield strength)

TECHNICAL FIELD

The present invention relates to a strengthened alpha brass and a methodfor manufacturing the same. The strengthened alpha brass is excellent inboth strength and formability, well balances within the strength and theformability, and has a certain level of stress relaxation properties.

BACKGROUND ART

Brass has been extensively used for forming electronic components suchas terminals or connectors or electromechanical components because brasshas relatively high mechanical strength, relatively good conductivity,and inexpensiveness. However, when components are subjected to severeforming, brass with lower temper grade has to be used to maintainnecessary formability. Such a shift to lower temper grade results inincrease of thickness of the material. As a result, the weight of thematerial increases and drawback in cost is caused.

When conventional brass is processed with high reduction for the purposeof obtaining materials having excellent strength, resulting materialshave deteriorated bend formability and poor toughness, so bending ofsuch materials tends to be difficult. Thus, conventional brass has sucha drawback. That is, when brass is processed to form connectors or thelike, severe bending is often applied. Therefore, for the purpose toprevent defects in required bending, materials having a 0.2% offsetyield strength less than 550 MPa are used in most cases. When a materialhaving a higher strength than this level is required, expensive phosphorbronze is generally selected. Furthermore, conventional brass does notexhibit excellent stress relaxation properties. In particular, whengrains are turned to be finer, the stress relaxation properties arefurther deteriorated, and which shows a serious problem in a practicaluse. Therefore, customer's demand is to avoid the deterioration of thestress relaxation properties due to making grains finer.

On the other hand, alpha brass is generally processed into plates orstrips by carrying out semi-continuous casting, hot rolling,sculpturing, subsequently cold rolling to have a certain thickness goodfor continuous annealing first. And then the brass strip is subjected tocontinuous annealing and pickling, cold rolling, continuous annealingand pickling, cold rolling, and cutting to finish a plate or a strip. Inthese processes, various manufacturing processes can be conducted. Forexample, annealing and rolling can be repeated according to thethickness, or the annealing can be a batch annealing. When a customerdemands annealing finish, the final rolling can be omitted. Processessuch as degreasing, pickling, leveling, cutting, or plating can befurther conducted between rolling and heat treatment or after rolling orheat treatment. Hereinafter, such conventional manufacturing processesare called “general manufacturing processes” in the presentspecification.

In the general manufacturing processes with a continuous annealing, theannealing condition is conducted in the range of 480-deg.C. to850-deg.C. as disclosed in Patent Document 1. On the other hand, a batchannealing is conducted in the range of 425-deg.C. to 600-deg.C. asdisclosed in Non-Patent Document 1. Furthermore, initial andintermediate annealings are conducted to prepare a grain size to be20-micron meter to 35-micron meter for the purpose of obtainingsufficient recrystallized structures and to decrease rolling pressurerequired. In this case, Vickers hardness (Hv) falls into the range of 60to 80. Then final annealing is conducted to arrange a grain size from5-micron meter to 60-micron meter according to applications, and aVickers hardness (Hv) falls into the range of 50 to 120. As mentionedabove, in the general manufacturing processes by carrying out the finalannealing, final cold rolling and the cutting are conducted to finishproduct plates or strips. Hereinafter, such products provided bycarrying out the final cold rolling are called “general materials orgeneral brass” in the present specification.

In order to increase strength of brass, using work hardening ispopularly known. Patent Document 2 discloses a method for obtaining highstrength by making grains finer and subjecting thus obtained brass tocold rolling for more increased strength. Methods to make grainstructure fine are disclosed in Non-Patent Documents 2 and 3.

In the disclosures, high reductions such as 92%, 91%, 80% or 78% areemployed, and subsequent annealing is conducted in a low temperaturerange for long hours such as, at 300-deg.C. for one hour, at 270-deg.C.for 7 hours, or at 230-deg.C. for 17 hours. Thus obtained strengths inthe annealed state are reported to be relatively high in annealed statesuch as a 0.2% offset yield strength of 379 MPa in the condition at300-deg.C. for one hour, a 0.2% offset yield strength of 399 MPa in thecondition at 270-deg.C. for 7 hours, and a 0.2% offset yield strength of534 MPa in the condition at 230-deg.C. for 17 hours.

Patent Document 2 also discloses a method for manufacturing brass withfine grains. However, the method disclosed in Patent Document 2 requiresrepeating rolling processes with a large reduction in a step-by-stepmanner. Therefore, this technique may be applied for manufacturingproducts with thin thicknesses. On the other hand, when relatively thickproducts are manufactured, difficulty may be caused to apply rollingprocesses with severe reduction for multiple times. Even condition ofits preliminary annealing is important, Patent Document 1 discloses justa final annealing and nothing is disclosed on it.

Non-Patent Document 4 discloses research results on making alpha plusbeta brass much stronger by making grains finer. Specifically, it isreported that a material having high strength and relatively good bendformability can be obtained by fine grained micro-duplex structure ofalpha phase and beta phase, and subjecting the brass to a lowtemperature annealing. It is also reported that stress relaxationproperties deteriorate as grains become finer, and the deterioration isslightly improved by a low temperature annealing.

[Patent Document 1]: Japanese Patent Laid-Open No. 53-32819

[Patent Document 2]: Japanese Patent Laid-Open No. 2004-292875

[Non-Patent Document 1]: Data Book for Copper and Copper Alloy Product,p. 19, issued by Japan Copper and Brass Association

[Non-Patent Document 2]: Copper and Copper Alloy 41, 1, and 29[Non-Patent Document 3]: Copper and Copper Alloy 43, 1, and 21

[Non-Patent Document 4]: Journal of the Japan Copper and Brass ResearchAssociation 39, 1, 128

[Non-Patent Document 5]: Data Book for Copper and Copper Alloy Product,p. 226, issued by Japan Copper and Brass Association

However, citing the case of carrying out annealing at 230-deg.C. for 17hours, which provides most excellent 0.2% offset yield strength in thedisclosures of Non-Patent Documents 2 and 3, relatively high strengthwith improved bend formability is obtained by arranging the materialafter cold rolling from the state of work hardened to the state of notenough softened by unfinished annealing. Thus obtained crystal structurehas ununiform re-crystallized state. Thus, this alloy has a decreased0.2% offset yield strength of about 534 MPa, and poor property.

On the other hand, the disclosure of Patent Document 2 suggests apreferred example of a method for manufacturing a brass with wellbalanced strength and bend formability. However, Patent Document 2discloses only one example suggesting balance between a specificstrength and bend formability. In addition, bend formability isevaluated by employing a bending with a bending axis across the rollingdirection which is considered a Good Way bend. So, the evaluation resultdoes not mean good bend formability with a bend axis along the rollingdirection which is considered a Bad Way bend.

In carrying out a typical method for manufacturing a strengthened alphabrass, it is widely known that making grains finer results in highstrength after annealing. It is also well known that brass having finegrains can be manufactured by subjecting brass to cold rolling with highreduction and repeating annealing in relatively low temperature rangesfor long hours. The manufacturing method disclosed in Patent Document 2requires combination of the process with high reduction for multipletimes. Therefore, it can be difficult to manufacture relatively thickproducts by this method. In addition, Patent Document 2 discloses just afew examples of preliminary annealing temperature before the finalre-crystallization annealing. Thus, such annealing conditions are nottaken as important ones.

In addition, in alpha plus beta brass, it is the fact that with finegrained micro-duplex structure of alpha phase and beta phase, the brassis much inferior to phosphor bronze in balance between 0.2% offset yieldstrength and bend formability.

As described above, though various suggestions are made on theoreticalmanufacturing methods, but managing of a manufacturing condition isdifficult and manufacturing conditions that permit industrial massproduction have not been found.

DISCLOSURE OF THE INVENTION

Then the present inventor has thoroughly investigated and foundmanufacturing conditions of providing fine grained structures inexcellent industrial productivity with less deviation in productsquality. That is, the present invention provides a method formanufacturing a strengthened alpha brass wherein the method is readilyapplicable in an industrial scale; the strengthened alpha brass hasformability required for the general alpha brass, but has excellentstrength than the general alpha brass; furthermore, the strengthenedalpha brass has a level of strength and formability equal to those of EHtemper grade of brass or more than that, those of temper H of phosphorbronze; and the strengthened alpha brass keeps a certain level of stressrelaxation resistance. The present invention also provides astrengthened alpha brass obtained by the method.

The present invention provides a method for manufacturing a strengthenedalpha brass having a composition of 63 wt % to 75 wt % copper,incidental impurities and the balance zinc, characterized in that abrass plate with a grain size from 1-micron meter to 2-micron meter isused as a starting plate material, the brass plate is cold rolled in 5%to 40% reduction to prepare a cold rolled brass plate, then the coldrolled brass plate is adjusted a 0.2% offset yield strength to be equalto or higher than 90% of its maximum value by subjecting a lowtemperature annealing.

The low temperature annealing is conducted at a temperature equal to orhigher than an annealing temperature at which the 0.2% offset yieldstrength exhibits the maximum value when estimated from relationshipwithin the 0.2% offset yield strength and annealing temperatures.

The brass plate with a grain size of 1-micron meter to 2-micron meterused as a starting plate material is preferably obtained by using a hotrolled brass plate or a brass plate with a grain size of 7-micron meterto 200-micron meter as a raw material, subjecting the material to a coldrolling process with a reduction of 80% to 95%, and then subjecting thematerial to a re-crystallization annealing to adjust a Vickers hardness(Hv) to be in the range of 130 to 170.

Alternatively, the brass plate with a grain size of 1-micron meter to2-micron meter used as a starting plate material is also preferablyobtained by using a hot rolled brass plate or a brass plate with a grainsize of 7-micron meter to 200-micron meter as a raw material, subjectingthe material to a cold rolling process with a reduction of 80% to 95%,then subjecting the material to a re-crystallization annealing to adjusta Vickers hardness (Hv) to be in the range of 130 to 170, subjecting thematerial to a cold rolling process with a reduction of 40% to 95%, andfurther subjecting the material to a re-crystallization annealing toadjust a Vickers hardness (Hv) to be in the range of 130 to 170.

Alternatively, the brass plate with a grain size of 1-micron meter to2-micron meter used as a starting plate material is also preferablyobtained by using brass plate with a grain size of 3-micron meter to6-micron meter as a raw material, subjecting the material to a coldrolling process with a reduction of 70% to 95%, and then subjecting thematerial to a re-crystallization annealing to adjust Vickers hardness(Hv) to be in the range of 130 to 170.

The re-crystallization annealing is preferably conducted in 370-deg.C.to 650-deg.C. for a continuous annealing, or in 255-deg.C. to 290-deg.C.for a batch annealing.

The present invention provides the strengthened alpha brass having acomposition of 63 wt % to 75 wt % copper, incidental impurities and thebalance zinc, characterized in that the strengthened alpha brass has atensile strength from 530 MPa to 790 MPa, a 0.2% offset yield strengthfrom 450 MPa to 750 MPa and a stress relaxation rate equal to or lessthan 52% after 100 hours at 120-deg.C.; and a minimum bend radius (MBR:mm) with which the strengthened alpha brass is 90 degree bent with abend axis along the rolling direction without causing cracks, a platethickness (t: mm) and a 0.2% offset yield strength (MPa) satisfy thefollowing Formula 4, where a value of right side of the Formula 4 isinterpreted as 0.3 when calculation result is equal to or less than 0.3.

MBR/t≦0.0125×σ_(0.2)−6.7 (σ_(0.2): 0.2% offset yield strength)  [Formula4]

It is also preferable that an Erichsen value (Er: mm) and the 0.2%offset yield strength (MPa) of the strengthened alpha brass preferablysatisfy the Formula 5.

Er≧−0.011×σ_(0.2)+13.7 (σ_(0.2): 0.27% offset yield strength)  [Formula5]

According to the present invention, a strengthened alpha brass which haswell balanced 0.2% offset yield strength and formability and a stressrelaxation rate equal to or less than a certain limit can be obtained.In addition, a strengthened alpha brass according to the presentinvention has excellent and stable properties, and it can be suitablymanufactured in industrial scale also.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a relation among temperatures of low temperature annealing,0.2% offset yield strength and stress relaxation rates obtained inExamples 1 and 2, and Comparative Examples 1 and 2; and

FIG. 2 shows softening curves obtained by carrying out preliminary testsin which reductions and annealing temperatures are changed in processingraw materials obtained by subjecting ingot 2 in Example 3 to hot rollingand then sculpturing the hot rolled material.

BEST MODE FOR CARRYING OUT THE INVENTION Manufacturing Method ofStrengthened Alpha Brass According to the Present Invention

A method for manufacturing a strengthened alpha brass according to thepresent invention is a method for manufacturing a strengthened alphabrass having a composition of 63 wt % to 75 wt % copper, incidentalimpurities and the balance zinc, characterized in that a brass platewith a grain size from 1-micron meter to 2-micron meter is used as astarting plate material, the brass plate is cold rolled in 5% to 40%reduction to prepare a cold rolled brass plate, then the cold rolledbrass plate is adjusted a 0.2% offset yield strength to be equal to orhigher than 90% of its maximum value by subjecting a low temperatureannealing.

First, the reason to define the composition of the strengthened alphabrass according to the present invention is described. When thecopper-zinc alloy has a copper content greater than 75 wt %, itsstrength level is inferior and performing of excessive increase in thestrength results in considerable tendency of causing deterioration ofbend formability. On the other hand, when the copper content is lessthan 63 wt %, beta phase appears and a single phase structure of alphaphase cannot be formed. In addition, as to incidental impurities, it isnecessary to give consideration to scrap materials which are used to cutcosts as is the case with wrought copper and copper alloy products. Feas an impurity influences re-crystallization temperature and thus Fe ispreferably 0.01 wt % or less. Sn as an impurity does not particularlygive a drawback. However, Sn exceeding 0.1 wt % has a good influence onstrength or corrosion resistance, the alloy having such Sn contentshould be treated separately. S as an impurity has a detrimental effecton hot formability, formability of final products such as wroughting ormachining. Therefore, S as an impurity is preferably limited to 0.003 wt% or less.

Because the starting plate material has uniform grain size from 1-micronmeter to 2-micron meter due to a re-crystallization process, obtainedcold rolled material may have more uniform grain size distribution as aresult. Then the brass plate is subjected to a cold rolling in 5% to 40%reduction to prepare a cold rolled brass plate. When the reduction inthe cold rolling process is less than 5%, 0.2% offset yield strengthdecreases even though a low temperature annealing described later may beconducted. On the other hand, when the reduction in the cold rollingprocess is greater than 40%, work hardening proceeds to result bendformability exceeds 3 in MBR/t while 0.2% offset yield strength isimproved. Thus, in this case, it may hard to obtain a strengthened alphabrass having good balance in mechanical properties.

By the way, 0.2% offset yield strength is used as an indicator ofmechanical strength of a strengthened alpha brass according to thepresent invention. The mechanical strengths of the general materials aregenerally represented by tensile strength and elongation. However, thetensile strength is a value calculated from a maximum load observeduntil break occurs. The value of the maximum load is data obtained whena tensile process has already been applied and alteration factorsinfluence sectional shape and physical properties. Therefore, thepresent inventor thinks that it is inappropriate to use tensile strengthas an indicator of formability. Then 0.2% offset yield strength, whichis mainly used as a standard for design is adopted as an indicator ofstrength, because with the indicator, properties themselves of materialsbefore being processed can be compared and evaluated.

Furthermore, when a material subjected to the final cold rolling issubjected to a low temperature annealing, as the temperature of the lowtemperature annealing increases, 0.2% offset yield strength increaseswith showing a moderate curve and then gradually decreases and thendrops rapidly. This is known as a low temperature annealing hardeningphenomenon. Then the temperature is limited so that 0.2% offset yieldstrength after the low temperature annealing is equal to or higher than90% of its maximum value of 0.2% offset yield strength obtained in thelow temperature annealing hardening phenomenon. This limitation is forthe purpose of suppressing decrease of strength. As for the temperaturethat provides the maximum value of 0.2% offset yield strength, the widthof the peak is narrow in the case of plotting along a temperature axisor a time axis when a reduction is low. However, when the reduction ishigh, a wide and mild peak can be obtained. Therefore, it is practicalto recognize a region in which 99% or higher of the maximum value isobtained according to heating conditions with which the maximum value isobtained rather than recognizing the condition with which the maximumvalue is obtained as a one point.

Then in view of dependency of the 0.2% offset yield strength onannealing temperatures, the low temperature annealing is preferablyconducted at a temperature equal to or higher than an annealingtemperature at which the 0.2% offset yield strength exhibits its maximumvalue. The final low temperature annealing conducted herein does notsimply denote an annealing that is conducted in low temperatures forstress relieving, but denotes a process involving the so-called lowtemperature annealing hardening phenomenon. On the other hand, thepresent inventor has found that a stress relaxation rate of about 55%after being cold rolled decreases to be a certain level as thetemperature increases from in the vicinity of annealing temperature thatprovides its maximum value of 0.2% offset yield strength. Therefore, asa condition to achieve the threshold of a stress relaxation rate equalto or less than 52%, the low temperature annealing temperature isrequired to be equal to or higher than an annealing temperature at whichthe 0.2% offset yield strength exhibits its maximum value.

The low temperature annealing is preferably conducted as continuousannealing rather than batch annealing. Furnace temperature is preferablyin the range of 250-deg.C. to 450-deg.C. Preferred time for passing aplate is 1 second to 10 seconds. The advantage of carrying out the lowtemperature annealing continuously is that cost reduction and assuringof quality stability are achieved easily. In addition, the lowtemperature annealing is a final annealing and after the low temperatureannealing is complete, the product is generally in the state of a strip.In the case of carrying out a batch annealing, the strip is introducedinto a heating furnace as a coil and heated with keeping the shape.Therefore, the strip curls and the curl in addition to strain caused byrolling are required to be leveled in the leveling process prior tousing the strip as a product. Thus carrying out effective leveling isdifficult. On the other hand, in the case of carrying out a continuousannealing, the plate materials are heated by running through a heatingzone and are wound as a coil after the low temperature annealing iscomplete. Therefore, thus obtained strips do not curl and flat stripsare easy to be obtained by subjecting the strips to a leveling process.

The brass plate with a grain size of 1-micron meter to 2-micron meterused as a starting plate material is preferably obtained by using a hotrolled brass plate or a brass plate with a grain size of 7-micron meterto 200-micron meter as a raw material, subjecting the material to a coldrolling process with a reduction of 80% to 95%, and then subjecting thematerial to a re-crystallization annealing to adjust a Vickers hardness(Hv) to be in the range of 130 to 170. The brass plate (annealedmaterial) with a large grain size used as a raw material also includeshot rolling finished materials. The grain size of hot rolled platematerials is 100-micron meter to 200-micron meter in the case of using asmall test scale rolling mill, but about 25-micron meter due to dynamicre-crystallization in the case of using an industrial scale hot rollingmill. When the cold rolling process is conducted with a reduction equalto or more than 80%, such a severe process provides a subgrain structureof about 1-micron meter even though the grain size before processing islarge. As a result, after the brass is subjected to subsequent steps,good 0.2% offset yield strength and bend formability, which is thetarget of the present invention, can be obtained. The present inventorhas confirmed that use of hot rolling finished materials obtained with asmall test scale rolling mill or an industrial scale hot rolling milldoes not result in difference in properties when the materials aresubjected to the cold rolling process with a reduction equal to or morethan 80%. Therefore, the lower limit of the reduction of the coldrolling process is defined as 80%. On the other hand, carrying out ofthe cold rolling process with a reduction greater than 95% may causecracks in the edge during the process, so which is not preferable.

The brass plate with a grain size of 1-micron meter to 2-micron meterused as a starting plate material is also preferably obtained by using ahot rolled brass plate or a brass plate with a grain size of 7-micronmeter to 200-micron meter as a raw material, subjecting the material toa cold rolling process with a reduction of 80% to 95%, then subjectingthe material to a re-crystallization annealing to adjust a Vickershardness (Hv) to be in the range of 130 to 170, subjecting the materialto a cold rolling process with a reduction of 40% to 95%, and furthersubjecting the material to a re-crystallization annealing to adjust aVickers hardness (Hv) to be in the range of 130 to 170. As mentionedabove, the plate obtained by using a hot rolled brass plate or a brassplate with a grain size of 7-micron meter to 200-micron meter as a rawmaterial, subjecting the material to a cold rolling process with areduction of 80% to 95%, then subjecting the material to are-crystallization annealing to adjust a Vickers hardness (Hv) to be inthe range of 130 to 170 has an average grain size from 1-micron meter to2-micron meter. In considering of this, when a reduction of 40% to 95%is adopted in the subsequent cold rolling process and the reduction islow but more than 40%, micro grains can be easily obtained by thesubsequent re-crystallization annealing. Then after the plate issubjected to subsequent processes, good 0.2% offset yield strength andbend formability can be obtained. On the other hand, carrying out thecold rolling process with a reduction greater than 95% may cause cracksin the edge during the process, and which is not preferable.

Alternatively, the brass plate with a grain size of 1-micron meter to2-micron meter used as a starting plate material is also preferablyobtained by using brass plate with a grain size of 3-micron meter to6-micron meter as a raw material, subjecting the material to a coldrolling process with a reduction of 70% to 95%, and then subjecting thematerial to a re-crystallization annealing to adjust Vickers hardness(Hv) to be in the range of 130 to 170. When the grain size before beingprocessed is larger than 6-micron meter, bend formability isdeteriorated because of poor fineness of grains even when the plate issubjected to the cold rolling process with a reduction of greater than70%. On the other hand, the average grain size of less than 3-micronmeter has disadvantage in some case, because this requires high rollingpressure even when the subsequent process is conducted with a reductionof 70%. In addition, carrying out the cold rolling process with areduction greater than 95% may cause cracks in the edge during theprocess, so it is not preferable.

The re-crystallization annealing is preferably conducted in 370-deg.C.to 650-deg.C. for a continuous annealing, or in 255-deg.C. to 290-deg.C.for a batch annealing. In the method for manufacturing a strengthenedalpha brass according to the present invention, the starting material issubjected to the cold rolling process, and then the finalre-crystallization annealing is conducted at the furnace temperature of370-deg.C. to 650-deg.C. for the continuous annealing. When the furnacetemperature is lower than 370-deg.C., obtained products may havedeteriorated formability, even re-crystallization is conducted bydecreasing the speed of passing the plates. On the other hand, when thefurnace temperature is higher than 650-deg.C., the grain size becomesununiform and grows greater than 2-micron meter. Then deterioratedformability is obtained even when cold rolled brass plates obtainedafter the cold roll process is subjected to the low temperatureannealing. The time for re-crystallization annealing is determineddepending on the performance of a furnace, a plate thickness and targetstrength. In the case of standard industrial equipment, the time is inthe range of 2 to 120 seconds. When appropriate time is actuallydetermined, the time is easily managed based on hardness so that Vickershardness (Hv) is in the range of 130 to 170, preferably in the range of135 to 160. When the Vickers hardness (Hv) is less than 130,re-crystallized grain size is so large that target physical propertiescannot be obtained even when the subsequent processes are conducted. Onthe other hand, when the Vickers hardness (Hv) is greater than 170, anobtained structure contains a higher ratio of remaining deformationstructure than a ratio of recrystallized structure. As a result, astrengthened alpha brass as a final product shows a deterioratedformability.

The advantage of carrying out the re-crystallization annealing by acontinuous process is that cost reduction and assuring of qualitystability are achieved easily. For a batch process, materialtemperatures tend to vary depending on positions in a furnace. Inaddition, after a batch annealing is conducted, value [0.2% offset yieldstrength]/[tensile strength] tend to be less than 80% after the finalre-crystallization annealing. On the other hand, when a continuousheating process is used, the value of [0.2% offset yieldstrength]/[tensile strength] become equal or higher than 80% after thefinal re-crystallization annealing, then finer grains can be obtained.Therefore, use of the continuous heating rather than the batch heatingprovides better balance between 0.2% offset yield strength andformability in a strengthened alpha brass obtained by carrying out thefinal cold rolling process and the low temperature annealing of themanufacturing method according to the present invention.

However, the batch annealing may be applicable when the plate thicknessis thick or no continuous annealing furnace is applicable. Industrially,the material is generally kept for 30 minutes to about 3 hours after thematerial temperature of a coil reaches a preset temperature. When thiskeeping time is adopted, preferred material temperature is 255-deg.C. to290-deg.C. When the material temperature of the coil is less than255-deg.C., grains obtained by re-crystallization for achieving targetstrength have ununiform sizes (when a grain size distribution chart ismade with plotting the grain size along an abscissa logarithmic axis,two or more peaks are observed), then bend formability is extremelyworsened even after low temperature annealing. On the other hand, whenthe material temperature of the coil is higher than 290-deg.C., grainsmay have uneven sizes and an average grain size might be large. Whensuch a plate is subjected to a cold rolling process followed by a lowtemperature annealing, product obtained may have deterioratedformability.

The present invention provides a strengthened alpha brass having acomposition of 63 wt % to 75 wt % copper, incidental impurities and thebalance zinc, characterized in that the strengthened alpha brass has atensile strength from 530 MPa to 790 MPa, a 0.2% offset yield strengthfrom 450 MPa to 750 MPa and a stress relaxation rate equal to or lessthan 52% after 100 hours at 120-deg.C.; and a minimum bend radius (MBR:mm) with which the strengthened alpha brass is 90 degree bent with abend axis along the rolling direction without causing cracks, a platethickness (t: mm) and a 0.2% offset yield strength (MPa) satisfy thefollowing Formula 6, where a value of right side of the Formula 6 isinterpreted as 0.3 when calculation result is equal to or less than 0.3.

MBR/t≦0.0125×σ_(0.2)−6.7 (σ_(0.2): 0.2% offset yield strength)  [Formula6]

As for an indicator of the strength of the strengthened alpha brass, the0.2% offset yield strength, which is mainly used as a standard fordesign is adopted as mentioned above.

As an indicator of evaluating formability of a copper alloy, mostly usedis a minimum bend radius (MBR: mm) with which cracks are not caused.Bend formability is important indicator in fabricating terminals or thelike. When referring to bend formability in the present application, itis to be understood that evaluation is conducted by the so-called Badway bending in which 90 degree bend test is conducted with a bend axisalong the rolling direction in various bending tests. When the so-calledGood way bending in which 90 degree bend test is conducted with a bendaxis across the rolling direction is used, better results are generallyobtained in evaluating brass. The present inventor considers that use ofthe Good way bending is inappropriate for comparing and selectingmanufacturing methods. Therefore, the present application employs onlythe Bad way bending as an evaluation method.

Now, the standard of the bend formability is described. When the MBR/tis equal to or less than 0.3, applying of almost any bending processdose not causes defects and shows no problem. On the other hand, theMBR/t equal to or less than 1.0 is often acceptable in consideration ofmaterials design. When the MBR/t is larger than 3.0, it is difficult tobend and applications of such materials are considerably restricted.There is a few conventional brass having a 0.2% offset yield strengthhigher than 550 MPa which provides the MBR/t equal to or larger than1.0. Brass having a 0.2% offset yield strength less than about 500 MPashow no problem in bend formability.

As for the stress relaxation rate, Japan Copper and Brass Associationdefine a test method (to be set as a cantilever and permanent deflectiondisplacement by bending is measured). Temperature applied to thestrengthened alpha brass according to the present invention is selectedto be 120° C. The test method defines its treatment time as 1000 hours,however, 100 hours are enough to evaluate difference and thus 100 hoursare selected. By using the method, the C2600 material and the C2680material, which are available in the market as materials for terminalsor connectors, were evaluated in terms of the stress relaxation rate.The results were 40%, 40%, 36%, 40%, and 48% to 52%. Thus, it has turnedout that the stress relaxation rate varies depending on temper gradesand grain sizes. At this time, the test pieces were evaluated within twoweeks from manufacturing in order to prevent possible influences of ageddeterioration on evaluation. Consequently, the present inventor definesthe strengthened alpha brass according to the present invention to havethe required stress relaxation rate with the threshold of 52% inconsideration of the fact that the above materials are actually used andcustomers do not like deterioration of the stress relaxation rate.

Furthermore, it is also preferable that an Erichsen value (Er: mm) andthe 0.2% offset yield strength (MPa) of the strengthened alpha brasspreferably satisfy the Formula 7.

Er≧−0.011×σ_(0.2)+13.7 (σ_(0.2): 0.2% offset yield strength)  [Formula7]

As mentioned above, when the MBR/t is equal to or less than 0.3, use ofalmost all bend formability dose not cause defects and show no problem.However, problems can be caused in crimping process of thick plates,bending of thick plates without bending R, bulging or the like. On theother hand, the present inventor has found that brass with fine grainshas excellent formability due to features of fine grains. In alpha brassin which grains are finished fine, when the 0.2% offset yield strengthis in the vicinity of 540 MPa or less, the minimum bend radius ofbending becomes zero. Therefore, the minimum bend radius cannot be usedas an indicator of formability that covers a wide range of strength.

Then the present application further uses an Erichsen value (Er: mm) asan additional indicator because the Erichsen value (Er: mm) is oftenused as an indicator of formability. In order to prove that thisselection is adequate, the present inventor collected 17 samples fromthe C2600 or C2680 material, with ½H, H and EH temper grade specified inJIS, and a 0.2% offset yield strength (MPa) and an Erichsen value (Er:mm) of each sample were evaluated. Then relation of the 0.2% offsetyield strength (MPa) and the Erichsen value (Er: mm) was examined andthe relation satisfy the Formula 8.

Er=−0.011×σ_(0.2)+12.7±0.5 (σ_(0.2): 0.2% offset yieldstrength)  [Formula 8]

It should be noted that the Erichsen value is a value obtained by thefollowing Erichsen Test. By using the Erichsen value, deep drawabilityof a sheet metal is judged.

(1) Standards of test equipment and test method: JIS B(2) Test method: a phi 90 plate, phi 27 dies (with a holddown to whichVaseline is applied), and D=20 hemisphere punch are used; When a crackthrough both sides is observed, the depth of the punch into the plate[Erichsen value (Er: mm)] is measured.

The present inventor has found that the Erichsen values (Er: mm) of thestrengthened alpha brass according to the present invention satisfy theFormula 9 whenever 0.2% offset yield strength fall within the range of450 MPa to 750 MPa; and the Erichsen values (Er: mm) of the strengthenedalpha brass according to the present invention are 0.5 mm or moresuperior to general materials having the same 0.2% offset yield strength(MPa). However, the Erichsen value (Er: mm) is not always used toevaluate formability in all applications. In particular, formability ofgeneral materials having MBR/t of exceeding 0.3 is recommended to beevaluated by not only Erichsen values (Er: mm) but also bendformability, which is used for direct evaluation.

Er≧−0.011×σ_(0.2)+13.7 (σ_(0.2): 0.2% offset yield strength)  [Formula9]

Next, physical properties of the strengthened alpha brass according tothe present invention are described in comparison with those of phosphorbronze. Then bend formability of phosphor bronze is described in orderto compare the bend formability with that of the strengthened alphabrass according to the present invention. Referring to data obtained bythe same evaluation method as the method used for evaluating 90 degreebend formability of the strengthened alpha brass according to thepresent invention in Non-Patent Document 5, bend formability of phosphorbronze can be represented by the following Formula 10 where a minimumbend radius without causing cracks is defined as MBR (mm), a platethickness is defined as t, and symbol [sigma]_(0.2) is used for 0.2%offset yield strength (MPa) as shown in the equation.

MBR/t≦0.0125×σ_(0.2)−7.0 (σ_(0.2): 0.2% offset yield strength)  [Formula10]

As for bend formability of phosphor bronze, according to the Formula 10,MBR/t becomes 0.3 or less when 0.2% offset yield strength is less than590 MPa. However, MBR/t becomes greater than 3 when 0.2% offset yieldstrength is greater than 800 MPa, and it is difficult to conduct bendingand phosphor bronze in this range is not practical. Actual measurementsof phosphor bronze sometimes do not satisfy the relationship. Forexample, MBR/t tends to become higher than the relationship on the sideof lower 0.2% offset yield strength and on the side of higher 0.2%offset yield strength. Based on what is mentioned above, physicalproperties of the strengthened alpha brass according to the presentinvention are described.

As for physical properties of the strengthened alpha brass according tothe present invention, a 0.2% offset yield strength is 450 MPa to 750MPa; and a minimum bend radius (MBR: mm) with which the strengthenedalpha brass is bent with a bend axis along the rolling direction withoutcausing cracks, a plate thickness (t: mm), and the 0.2% offset yieldstrength satisfy the following Formula 11, where a value of right sideof the formula 11 is interpreted as 0.3 when a calculated result of theright side of the Formula 11 is equal to or less than 0.3. Then thefollowing Formula 11 is shifted by 0.3 from the following Formula 12 ofphosphor bronze.

MBR/t≦0.0125×σ_(0.2)−6.7 (σ_(0.2): 0.2% offset yield strength)  [Formula11]

MBR/t≦0.0125×σ_(0.2)−7.0 (σ_(0.2): 0.2% offset yield strength)  [Formula12]

Therefore, the strengthened alpha brass according to the presentinvention that satisfies the Formula 11 is understood to have the samelevel of bend formability as phosphor bronze in balance of 0.2% offsetyield strength and the bend formability in consideration of presence ofquality deviation of phosphor bronze. When the relationship between avalue of MBR/t and a value of 0.2% offset yield strength does notsatisfy the Formula 11, bend formability is poor. The reason forinterpreting a calculated result of MBR/t as 0.3 when the calculatedresult is equal to or less than 0.3 is that MBR/t tends to become higherthan the relationship on the side of lower 0.2% offset yield strength aswith phosphor bronze; when the calculated result is equal to or lessthan 0.3, bend formability hardly show a problem; and measurements caninclude certain deviation.

A strengthened alpha brass that satisfies the following Formula 13 interms of bend formability (MBR/t) and the following Formula 14 in termsof Erichsen value (Er) has a structure mostly derived from arecrystallized structure with an average grain size equal to or lessthan 2-micron meter. Such a strengthened alpha brass preferably has arecovery structure described later and has an average grain size equalto or less than 2-micron meter at the time of re-crystallization.

MBR/t≦0.0125×σ_(0.2)−6.7 (σ_(0.2): 0.2% offset yield strength)  [Formula13]

Er≧−0.011×σ_(0.2)+13.7 (σ_(0.2): 0.2% offset yield strength)  [Formula14]

Furthermore, it is preferable that the strengthened alpha brass has arecovery structure after the low temperature annealing, 80% or more of avalue of [0.2% offset yield strength]/[tensile strength] and an averagegrain size equal to or less than 1.5-micron meter at the time ofre-crystallization, because in such case, the constant 13.7 in theFormula 15 may be changed to 14.2, and the constant 6.7 in the Formula16 may be changed to 7.1.

Er≧−0.011×σ_(0.2)+13.7 (σ_(0.2): 0.2% offset yield strength)  [Formula15]

MBR/t≦0.0125×σ_(0.2)−6.7 (σ_(0.2): 0.2% offset yield strength)  [Formula16]

The construction of the grain structure of the strengthened alpha brassaccording to the present invention is described so far. The grain sizeof recrystallized grains can be measured by an intercept method or aphotograph comparison method with an optical microscope with highmagnification or a scanning electron microscope after electrolyticetching. Change of the structure due to a low temperature annealing canbe identified considerably by observation with SEM-EBSP. In particular,when an image processing is conducted so that a portion whose imagequality value is equal to or less than a certain value (strain relievingis a level equal to or less than a certain value) is represented asblack, recovered grains are recognized as bright grains. As recoveryproceeds, the outline of a grain becomes smooth, and recrystallizedgrains are recognized as bright grains with annealing twin. Thestructure of a strengthened alpha brass with good bend formability is amicro structure composed of mixture of grains in which strains arerelieved by a low temperature annealing (recovered grains orrecrystallized grains) and grains in which strains are not relieved.This micro structure is similar to the fine grained micro-duplexstructure. The micro structure is considered to induce heterogeneoussliding and improve bend formability. Improvement of stress relaxationresistance by a low temperature annealing corresponds to increase of aratio of area of recovered grains or recrystallized grains. Therefore,structural change is essential to assure good stress relaxationresistance. It should be noted that the strengthened alpha brassaccording to the present invention has fine grains with a level of1-micron meter to 2-micron meter, and thus has excellent fatiguestrength, excellent stress-corrosion cracking resistance, and has asmall deflection coefficient.

As mentioned above, in order to obtain a strengthened alpha brass by themanufacturing method according to the present invention, fine anduniform grains are obtained at the time of the final re-crystallizationannealing, rolling is conducted to obtain a target strength, and a lowtemperature annealing is conducted to obtain a aiming grain structure inwhich strains are relieved partially. It should be noted that a certainlevel of management is necessary for cold reduction prior to the finalre-crystallization annealing and grain sizes prior to the cold rollingfor the purpose of obtaining fine and uniform grains.

EXAMPLES

Hereinafter, the present invention is described further in detail withreferring to Examples. The chemical compositions of brass ingots usedfor manufacturing and evaluation in Examples and Comparative Examplesare shown in Table 1. Ingots 1 to 6 herein are samples obtained bysemicontinuous casting in casting plant where manufacturing isconducted. Each of ingots 7, 8 and 9 are obtained by melting with afurnace in a laboratory and casting an ingot to have a size by 30 mm by100 mm by 200 mm with a metal mold.

TABLE 1 Component Composition (wt %) Ingot No. Cu Fe Pb Sn S Zn 1 65.20.002 0.000 0.003 0.001 Balance 2 69.9 0.004 0.000 0.003 0.000 Balance 369.3 0.002 0.000 0.003 0.000 Balance 4 65.4 0.002 0.002 0.001 0.003Balance 5 70.0 0.003 0.000 0.002 0.000 Balance 6 69.9 0.002 0.002 0.0010.001 Balance 7 74.2 0.006 0.000 0.002 0.000 Balance 8 68.9 0.001 0.0000.001 0.000 Balance 9 65.9 0.001 0.000 0.001 0.000 Balance

As is evident from Table 1, Ingots 1 to 9 satisfy the state of thepresent invention being a composition of wt % to 74.2 wt % copper,incidental impurities, with the balance zinc. Furthermore, in thefollowing Examples, any one of the Ingots shown in Table 1 is used andmanufacturing conditions comprising following steps (a) to (e) shown inTable 2 are applied to prepare brass strips.

(a) Preparation of raw material

(b) Cold rolling

(c) Re-crystallization annealing

(d) Final cold rolling

(e) Low temperature annealing

TABLE 2 Annealing (c) Prior to Final Preliminary Annealing (a) ProcessReduction Temperature Hardness Reduction Hardness (%) in (° C.) of LowIngot Temperature (Hv) after (%) in Cold Temperature (Hv) after FinalCold Temperature Samples No. (° C.) Annealing Rolling (b) (° C.)Annealing Rolling (d) Annealing (e) Examples 1-1 1 510 136 72 600 152 10280 1-2 1 510 136 72 600 152 10 340 1-3 1 510 136 72 600 152 10 420 2-11 510 136 72 600 152 24 260 2-2 1 510 136 72 600 152 24 280 2-3 1 510136 72 600 152 24 300 2-4 1 510 136 72 600 152 24 340 3 2 Sculpturingafter Hot Rolling 95 430 151 10 320 Comparative 1-4 1 510 136 72 600 15210 None Examples 1-5 1 510 136 72 600 152 10 240 1-6 1 510 136 72 600152 10 260 2-5 1 510 136 72 600 152 24 None 2-6 1 510 136 72 600 152 24240 2-7 1 510 136 72 600 152 24 420

Examples 1 and 2

The Ingot 1 obtained above was hot rolled, sculptured, cold rolled, andpreliminary annealed to obtain a raw material with a thickness of 1.8mm. Preparation of the raw material and a starting plate material 1 wereconducted in a production line at the manufacturing site until theannealing (c) prior to the final cold rolling. Process conditionsapplied to Examples 1 and 2 are shown in Table 2 with Example 3 incomparison with Comparative Examples 1 and 2. In Table 2, thepreliminary annealing (a: annealing prior to the re-crystallizationannealing prior to the final cold rolling) and the annealing prior tothe final process (c: annealing prior to the final cold rolling) arecontinuous annealings conducted in the production lines at themanufacturing site as mentioned above. The temperatures mentioned aboveare preset temperatures of furnaces. In this way, common starting platematerial 1 was used for the process until the re-crystallizationannealing prior to the final cold rolling.

Examples 1

In these Examples, the starting plate materials 1 obtained above weresubjected to cold rolling (d) with a reduction of 10% by using alaboratory cold rolling mill to prepare cold rolled brass plates, andfurther subjected to a low temperature annealing (e) in a salt bath.Annealing time in the salt bath was set to be short time of 2 secondsfor the purpose of carrying out the annealing to be similar to acontinuous annealing. Temperatures in the salt bath of Examples 1-1,1-2, and 1-3 were 280-deg.C., 340-deg.C., and 420-deg.C. respectively.Physical properties of strengthened alpha brass obtained were evaluated.As a result, tensile strengths were 532 MPa to 556 MPa, 0.2% offsetyield strength were 458 MPa to 504 MPa, Erichsen values (valuecalculated from 0.2% offset yield strength) were 8.6 mm (8.3 mm) to 8.8mm (8.2 mm) and stress relaxation rates were 47% to 51%. Thus, thephysical properties satisfied the target values. Details are shown inTable 3.

Examples 2

In these Examples, the starting plate materials 1 as with Examples weresubjected to cold rolling (d) with a reduction of 24% by using alaboratory cold rolling mill to prepare cold rolled brass plates, andfurther subjected to a low temperature annealing (e) in a salt bath.Annealing time in the salt bath was set to be short time of 2 secondsfor the purpose of carrying out the annealing to be similar to acontinuous annealing. Temperatures in the salt bath of Examples 2-1,2-2, 2-3 and 2-4 were 260-deg.C., 280-deg.C., 300-deg.C., and 340-deg.C.respectively. Physical properties of strengthened alpha brass obtainedwere evaluated. As a result, tensile strengths were 667 MPa to 680 MPa,0.2% offset yield strength were 622 MPa to 638 MPa, Erichsen values(value calculated from 0.2% offset yield strength) were 6.8 mm (6.7 mm)to 8.1 mm (6.9 mm) and stress relaxation rates were 41% to 52%. Thus,the physical properties satisfied the target values. MBR/t values (valuecalculated from 0.2% offset yield strength), which are indicators ofbend formability, were 0.5 (1.1) to 0.6 (1.3). Details are shown inTable 3 with Examples 1. Then influences of temperature settings of thelow temperature annealing on 0.2% offset yield strength and stressrelaxation rates are shown in FIG. 1. The grains obtained in Examplesand Comparative Examples after the annealing (b) prior to the finalprocess had a size of about 2-micron meter, and the grains after thefinal cold rolling (d) had a size of 1-micron meter.

TABLE 3 Erichsen Value (mm) MBR/t 0.2% Offset Calculated CalculatedTensile Yield Actual from 0.2% Stress Actual from 0.2% Strength StrengthElongation Measure- Offset Yield Relaxation Measure- Offset YieldSamples (MPa) (MPa) (%) ments Strength Rate (%) ments Strength Examples1-1 556 504 21 8.8 8.2 51 1-2 549 492 22 8.6 8.3 47 1-3 532 458 27 8.78.7 48 2-1 672 632 4 7.2 6.7 52 0.6 1.2 2-2 680 638 3 7.0 6.7 51 0.6 1.32-3 673 633 3 6.8 6.7 49 0.6 1.2 2-4 667 622 6 8.1 6.9 41 0.5 1.1 3 557499 22 8.8 8.2 49 Comparative 1-4 555 496 21 8.7 8.2 53 Examples 1-5 547495 23 9.1 8.3 54 1-6 559 499 20 8.5 8.2 53 2-5 653 608 7 7.3 7.0 59 0.60.9 2-6 670 631 4 7.4 6.8 55 0.6 1.2 2-7 613 559 13 8.2 7.6 42 0.1 0.3When MBR/t calculated from 0.2% offset yield strength is less than 0.3,MBR/t is defined as 0.3.

Examples 3

In this Example, the Ingot 2 was used. The Ingot 2 was hot rolled andsculptured (a) to obtain a raw material with a thickness of 11.5 mm.Preliminary tests were conducted in which reductions and annealingtemperatures were changed to obtain softening curves. Annealing time inthe salt bath was 10 seconds. Thus, obtained softening curves are shownin FIG. 2. According to FIG. 2, annealed materials afterre-crystallization stably had Vickers hardness (Hv) of about 150, exceptfor materials with a reduction of 70%. According to observation ofgrains with an optical microscope, materials with a reduction of 70% hada deformation structure up to 430-deg.C., and the material had a grainstructure in which grains with a size up to 10-micron meter and grainswith a size less than 3-micron meter are mixed at condition 450-deg.C.On the other hand, a material which was cold rolled with anotherreduction and annealed at 430-deg.C., the material had a grain size ofabout 2-micron meter.

Based on the results of the preliminary tests, a plate material wassubjected to cold rolling (b) with reduction of 95% by using alaboratory cold rolling mill, and further subjected to are-crystallization annealing (c) in a salt bath at 430-deg.C. to obtaina starting plate material. After that, this material was subjected tocold rolling (d) with a reduction of 10% down to thickness of 0.52 mm toprepare a cold rolled brass plate, and further subjected to a lowtemperature annealing (e) in a salt bath at 320-deg.C. for 2 seconds. Asfor thus obtained strengthened alpha brass, the tensile strength was 557MPa, the 0.2% offset yield strength was 499 MPa, The Erichsen value(value calculated from 0.2% offset yield strength) was 8.8 mm (8.2 mm)and the stress relaxation rate was 49%. Thus excellent physicalproperties were obtained with carrying out re-crystallization annealingonly once. The manufacturing conditions are shown in Table 2, and theresults are shown in Table 3 with those of Examples 1 and 2.

Examples 4 to 8

In these Examples, the Ingots 2 to 6 were used in the Examplesrespectively as shown in Table 4. The entire process from casting to thefinal cold rolling was conducted by using a production line at themanufacturing site. First, a material plate with at thickness of 11.5 mmafter hot rolling and sculpturing was subjected to cold rolling with areduction of 84% to have a thickness of 1.8 mm. The plate material wassubjected to preliminary annealing shown in Table 4 (a: annealing of arough-rolled strip) to prepare a raw material. After that, the rawmaterial was subjected again to cold rolling (b) and to a finalre-crystallization annealing (c) to obtain a starting plate material.Among Examples, cold rolling and re-crystallization annealing wereconducted prior to the final re-crystallization annealing in Example 8(described in the upper portion of Table 4). The materials weresubjected to the final cold rolling (d) to prepare cold rolled brassplates, and then subjected to a low temperature annealing (e) to provideproducts. As for conditions of the low temperature annealing, theannealing was conducted as a batch process one hour at 200-deg.C. inExample 4 while continuous annealing was conducted at a furnacetemperature of 420-deg.C. in other Examples. These setting of continuousannealing conditions were intended to obtain the maximum value of 0.2%offset yield strength. As a result of evaluating strengthened alphabrass, tensile strengths were 534 MPa to 776 MPa, 0.2% offset yieldstrength were 470 MPa to 727 MPa, Erichsen values (value calculated from0.2% offset yield strength) were 6.2 mm (6.1 mm) to mm (8.5 mm) andstress relaxation rates were 40% to 51%. Thus, the physical propertiessatisfied the target values. MBR/t values (value calculated from 0.2%offset yield strength), which are indicators of bend formability, were0.0 (0.3) to 1.9 (2.4). Details are shown in Table 5. In this table,when MBR/t calculated from 0.2% offset yield strength is less than 0.3,MBR/t is defined as 0.3.

TABLE 4 Final Re-crystallization Annealing Preliminary Annealing (a) (c)Reduction Temperature (° C.) Grain Reduction Annealing Grain (%) in ofLow Ingot Temperature Hardness Size (%) in Cold Temperature HardnessSize Final Cold Temperature Samples No. (° C.) (Hv) (μm) Rolling (b) (°C.) (Hv) (μm) Rolling (d) Annealing (e:*) Examples 4 2 640 117 4 78 420148 1 33 200-B 5 3 530 144 2 56 550 147 1 36 420-C 6 4 510 137 2 78 600156 1 7 420-C 7 5 530 140 2 78 550 168 1 14 420-C 8 6 530 140 2 63 550145 2 17 420-C 75 500 152 1 9 the same conditions as Example 5 40 420141 2 30 280-B 10 7 640 121 5 78 270 149 2 25 205-B 11 8 640 118 5 78270 146 2 25 205-B 12 9 640 119 5 78 270 140 2 25 205-B Note*: B at theend of the temperature of low temperature annealing means an materialtemperature of brass in a batch annealing, and C means an furnacetemperature in a continuous annealing.

TABLE 5 Erichsen value (mm) MBR/t 0.2% Offset Calculated CalculatedTensile Yield Actual from 0.2% Stress Actual from 0.2% Strength StrengthElongation Measure- Offset Yield Relaxation Measure- Offset YieldSamples (MPa) (MPa) (%) ments Strength Rate (%) ments Strength Examples4 776 727 0.5 6.4 5.7 41 1.9 2.4 5 699 688 1.8 6.2 6.1 40 1.3 1.9 6 534470 24 9.6 8.5 51 0.0 0.3 7 598 537 17 8.5 7.8 49 0.0 0.3 8 603 554 12.57.9 7.6 38 0.1 0.3 9 651 601 6.9 7.2 7.1 38 0.6 0.8 When MBR/tcalculated from 0.2% offset yield strength is less than 0.3, MBR/t isdefined as 0.3.

Example 9

In this Example, the sample piece after the preliminary annealing (a:annealing of a rough-rolled strip) in Example 5 was used as a rawmaterial. The raw material was subjected to cold rolling (b) with areduction of 40% by using a laboratory cold rolling mill. This strip wassubjected to a final re-crystallization annealing (c) in a salt bath at420-deg.C. for 10 seconds to obtain a starting plate material. Afterthat, the material was subjected to the final cold rolling (d) with areduction of 30% to prepare a cold rolled brass plate, and thensubjected to a low temperature annealing (e) at 280-deg.C. for 10seconds. As for thus obtained strengthened alpha brass, tensile strengthwas 651 MPa, 0.2% offset yield strength was 601 MPa, elongation was6.9%, Erichsen value (value calculated from 0.2% offset yield strength)was 7.2 mm (7.1 mm) and MBR/t (value calculated from 0.2% offset yieldstrength) was 0.6 (0.8). Thus, excellent physical properties wereobtained.

Examples 10 to 12

In these Examples, the Ingots 7 to 9 were used in the Examplesrespectively as shown in Table 4. In a laboratory, each of the Ingotswas subjected to hot rolling causing the grain size to be 0.15 mm,subsequently subjected to cold rolling with a reduction of 86%, and thensubjected to re-crystallization annealing (a) with setting conditions sothat the grain size to be 5-micron meter. Such a raw material wasfurther subjected to cold rolling (b) with a reduction of 78%. Thusobtained plate material was subjected to a final re-crystallizationannealing (c) for 2 hours with a material temperature of 270-deg.C. toobtain a starting plate material. The material was subjected to thefinal cold rolling (d) with a reduction of 25% to prepare cold rolledbrass plates, and then subjected to final re-crystallization annealing(e) at a material temperature of 205-deg.C. The low temperatureannealing at this time were conducted by using a muffle furnace withmeasuring material temperatures. Physical properties of thus obtainedstrengthened alpha brass with at thickness of 0.3 mm were evaluated.Tensile strengths were 671 MPa to 681 MPa, 0.2% offset yield strengthwere 629 MPa to 640 MPa, Erichsen values (value calculated from 0.2%offset yield strength) were 6.7 mm (6.7 mm) to 7.0 mm mm) and stressrelaxation rates were 40% to 41%. Thus the physical properties satisfiedthe target values. MBR/t values (value calculated from 0.2% offset yieldstrength), which are indicators of bend formability, were 0.9 (1.2) to0.9 (1.3). Details are shown in Table 6.

TABLE 6 Erichsen Value (mm) MBR/t 0.2% Offset Calculated CalculatedTensile Yield Actual from 0.2% Stress Actual from 0.2% Grain IngotStrength Strength Elongation Measure- Offset Yield Relaxation Measure-Offset Yield Size Samples No. (MPa) (MPa) (%) ments Strength Rate (%)ments Strength (μm) Examples 10 7 671 629 2.0 7.0 6.8 40 0.9 1.2 2 11 8681 640 1.0 6.7 6.7 41 0.9 1.3 2 12 9 672 633 1.0 7.0 6.7 41 0.9 1.2 2Comparative 7 666 609 2.0 6.0 7.0 49 2.4 0.9 15 Example 3

Comparative Examples 1 and 2

These Comparative Examples were conducted as with Examples 1 and 2except that conditions of the final low temperature annealing werechanged. The conditions are shown in Table 2.

Comparative Examples 1

the starting plate material 1 as with Example 1 was subjected to coldrolling with a reduction of 10% by using a laboratory cold rolling mill,and further subjected to a low temperature annealing in a salt bath. InComparative Example 1-4, the low temperature annealing was notconducted. In Comparative Examples 1-5 and 1-6, annealing times in asalt bath were set to short times of 2 seconds as with Examples, andannealing temperatures were set to 240-deg.C. and 260-deg.C.respectively. Physical properties of strengthened alpha brass obtainedwere evaluated. As a result, tensile strengths were 547 MPa to 559 MPa,0.2% offset yield strength were 495 MPa to 499 MPa, Erichsen values(value calculated from 0.2% offset yield strength) were 8.5 mm (8.2 mm)to 9.1 mm (8.3 mm) and stress relaxation rates were 53% to 54%. Thus,the stress relaxation rates do not satisfy the target values. Detailsare shown in Table 3 with Examples 1.

Comparative Examples 2

the starting plate material 1 as with Examples 1 and 2 was subjected tocold rolling with a reduction of 24% by using a laboratory cold rollingmill, and further subjected to a low temperature annealing in a saltbath. In Comparative Example 2-5, the low temperature annealing was notconducted. In Comparative Examples 2-6 and 2-7, annealing times in asalt bath were set to short times of 2 seconds as with Examples, andannealing temperatures were set to 240-deg.C. and 420-deg.C.respectively. Physical properties of strengthened alpha brass obtainedwere evaluated. As a result, tensile strengths were 613 MPa to 670 MPa,0.2% offset yield strength were 559 MPa to 631 MPa, Erichsen values(value calculated from 0.2% offset yield strength) were 7.3 mm (7.0 mm)to 8.2 mm (7.6 mm) and stress relaxation rates were 42% to 59%. Thus,the 0.2% offset yield strength or the stress relaxation rates do notsatisfy the target values. MBR/t values (value calculated from 0.2%offset yield strength), which are indicators of bend formability, were0.1 (0.29) to 0.6 (1.19). Details are shown in Table 3 with Examples 1.

Comparative Example 3

In this Comparative Example, the Ingot 7 was used and a sample having a0.2% offset yield strength as with Example 11 was prepared by carryingout processes similar to conventional processes in a laboratory. Thatis, the sample was subjected to hot rolling, cold rolling, annealing sothat grain size to be 35-micron meter, and then cold rolling with areduction of 53%. Then re-crystallization annealing was conducted with asalt bath at 650-deg.C. for 20 seconds so that pseudo-continuousannealing in conventional processes was conducted. As a result, grainsize after the final re-crystallization annealing was 15-micron meter.Then the sample was subjected to a final cold rolling with a reductionof 65%. Physical properties of strengthened alpha brass obtained wereevaluated. As a result, tensile strength was 666 MPa, 0.2% offset yieldstrength was 609 MPa, Erichsen value (value calculated from 0.2% offsetyield strength) was 6.0 mm (7.0 mm) and stress relaxation rate was 49%.Thus, the Erichsen value does not satisfy the target value. MBR/t value(value calculated from 0.2% offset yield strength), which is anindicator of bend formability, was poor of 2.4 (0.9). Details are shownin Table 6 with Examples 10 to 12. As is evident from Table 6, when thereduction in the final rolling is increased to increase 0.2% offsetyield strength, formability is considerably deteriorated.

Reference Examples

As Reference Examples, physical properties of commercially available Hand EH temper grade of C2680 material (Cu/Zn: 65%/35%), and H tempergrade of C2600 material (Cu/Zn: 70%/30%) were evaluated. The brass ofReference Examples was subjected to re-crystallization annealing andthen final cold rolling with a reduction of 25%, 17% and 35%respectively. The brass of Reference Examples was not subjected to a lowtemperature annealing. As for evaluation results, tensile strengths were486 MPa to 567 MPa, 0.2% offset yield strength were 437 MPa to 524 MPa,stress relaxation rates were 36% to 52%, and Erichsen values (Er) were6.9 mm to 8.3 mm. Details are shown in Table 7. Reference Example 1 doesnot satisfy mechanical strength and an Erichsen value (Er) of thefollowing Formula 18 according to the present invention. ReferenceExample 2 had finer grains than Reference Example 1 and thus had arelatively higher Erichsen value. However, Reference Example 2 does notsatisfy the following Formula 18 and had a rather large stressrelaxation rate. Reference Example 3 satisfies target mechanicalstrength and a stress relaxation rate. However, Reference Example 3 doesnot satisfy the following Formula 17 in terms of MBR/t and the followingFormula 18 in terms of an Erichsen value (Er).

TABLE 7 Erichsen value (mm) MBR/t 0.2% Offset Calculated CalculatedTensile Yield Actual from 0.2% Stress Actual from 0.2% Grain TemperStrength Strength Elongation Measure- Offset Yield Relaxation Measure-Offset Yield Size Samples Alloys grade (MPa) (MPa) (%) ments StrengthRate (%) ments Strength (μm) Reference 1 C2680 H 486 437 22 7.6 8.9 400.0 0.3 11 Examples 2 C2600 H 535 485 21 8.3 8.4 52 0.0 0.3 5 3 C2680 EH567 524 10 6.9 7.9 36 0.5 0.3 15 When MBR/t calculated from 0.2% offsetyield strength is less than 0.3, MBR/t is defined as 0.3.

MBR/t≦0.0125×σ_(0.2)−6.7 (σ_(0.2): 0.2% offset yield strength)  [Formula17]

Er≧−0.011×σ_(0.2)+13.7 (σ_(0.2): 0.2% offset yield strength)  [Formula18]

INDUSTRIAL APPLICABILITY

The strengthened alpha brass according to the present invention has ageneral alpha brass composition in view of composition. However, bycarrying out proper rolling processes and heat treatments in themanufacturing method according to the present invention, obtained alphabrass exhibits excellent balance between strength and formability, whichhas never achieved in conventional alpha brass and the balance is at thesame level or better than phosphor bronze. Such a strengthened alphabrass is suitable for forming electronic components such as connectorsor electromechanical components and can be provided as an inexpensivematerial.

Furthermore, the method for manufacturing a strengthened alpha brassaccording to the present invention can be conducted with conventionallyused rolling production lines without changing the lines. Therefore, useof the method does not require additional investment for equipments, andstrengthened alpha brass of excellent quality can be manufacturedefficiently in an industrial scale.

1. A method for manufacturing a strengthened alpha brass having acomposition of 63 wt % to 75 wt % copper, incidental impurities and thebalance zinc, characterized in that a brass plate with a grain size from1-micron meter to 2-micron meter is used as a starting plate material,the brass plate is cold rolled in 5% to 40% reduction to prepare a coldrolled brass plate, then the cold rolled brass plate is adjusted a 0.2%offset yield strength to be equal to or higher than 90% of its maximumvalue by subjecting a low temperature annealing.
 2. The method formanufacturing a strengthened alpha brass according to claim 1, whereinthe low temperature annealing is conducted at a temperature equal to orhigher than an annealing temperature at which the 0.2% offset yieldstrength exhibits the maximum value when estimated from relationshipwithin 0.2% offset yield strength and annealing temperatures.
 3. Themethod for manufacturing a strengthened alpha brass according to claim1, wherein the brass plate with a grain size of 1-micron meter to2-micron meter used as a starting plate material is obtained by using ahot rolled brass plate or a brass plate with a grain size of 7-micronmeter to 200-micron meter as a raw material, subjecting the material toa cold rolling process with a reduction of 80% to 95%, and thensubjecting the material to a re-crystallization annealing to adjust aVickers hardness (Hv) to be in the range of 130 to
 170. 4. The methodfor manufacturing a strengthened alpha brass according to claim 1,wherein the brass plate with a grain size of 1-micron meter to 2-micronmeter used as a starting plate material is obtained by using a hotrolled brass plate or a brass plate with a grain size of 7-micron meterto 200-micron meter as a raw material, subjecting the material to a coldrolling process with a reduction of 80% to 95%, then subjecting thematerial to a re-crystallization annealing to adjust a Vickers hardness(Hv) to be in the range of 130 to 170, subjecting the material to a coldrolling process with a reduction of 40% to 95%, and further subjectingthe material to a re-crystallization annealing to adjust a Vickershardness (Hv) to be in the range of 130 to
 170. 5. The method formanufacturing a strengthened alpha brass according to claim 1, whereinthe brass plate with a grain size of 1-micron meter to 2-micron meterused as a starting plate material is obtained by using a brass platewith grain size of 3-micron meter to 6-micron meter as a raw material,subjecting the material to a cold rolling process with a reduction of70% to 95%, and then subjecting the material to a re-crystallizationannealing to adjust Vickers hardness (Hv) to be in the range of 130 to170.
 6. The method for manufacturing a strengthened alpha brassaccording to claim 3, wherein the re-crystallization annealing isconducted in 370-deg.C. to 650-deg.C. for a continuous annealing, or in255-deg.C. to 290-deg.C. for a batch annealing.
 7. A strengthened alphabrass obtained by the manufacturing method according to claim 1, thestrengthened alpha brass having a composition of 63 wt % to 75 wt %copper, incidental impurities and the balance zinc, characterized inthat the strengthened alpha brass has a tensile strength from 530 MPa to790 MPa, a 0.2% offset yield strength from 450 MPa to 750 MPa and astress relaxation rate equal to or less than 52% after 100 hours at120-deg.C.; and a minimum bend radius (MBR: mm) with which thestrengthened alpha brass is 90 degree bent with a bend axis along therolling direction without causing cracks, a plate thickness (t: mm) anda 0.2% offset yield strength (MPa) satisfy the following Formula 1,where a value of right side of the Formula 1 is interpreted as 0.3 whencalculation result is equal to or less than 0.3.MBR/t≦0.0125×σ_(0.2)−7.0 (σ_(0.2): 0.2% offset yield strength)  [Formula1]
 8. A strengthened alpha brass obtained by the manufacturing methodaccording to claim 1, the strengthened alpha brass having a compositionof 63 wt % to 75 wt % copper, incidental impurities and the balancezinc, characterized in that the strengthened alpha brass has a tensilestrength of from 530 MPa to 790 MPa, a 0.2% offset yield strength offrom 450 MPa to 750 MPa, a stress relaxation rate equal to or less than52% at 120-deg.C. for 100 hours; and a minimum bend radius (MBR: mm)with which the strengthened alpha brass is 90 degree bent with a bendaxis along the rolling direction without causing cracks, a platethickness (t: mm) and a 0.2% offset yield strength (MPa) satisfy thefollowing Formula 1, where a value of right side of the Formula 1 isinterpreted as 0.3 when calculation result is equal to or less than 0.3;and an Erichsen value (Er: mm) and the 0.2% offset yield strength (MPa)satisfy the following Formula 3.MBR/t≦0.0125×σ_(0.2)−6.7 (σ_(0.2): 0.2% offset yield strength)  [Formula2]Er≧−0.011×ν_(0.2)+13.7 (σ_(0.2): 0.2% offset yield strength)  [Formula3]